The importance of sputter-coated glass layer systems for achieving solar management properties in many types of glass articles, such as architectural windows and doors, is now well established in commerce. In addition, the importance of using such layer systems in insulating glass units (known as "IG" units in the art) is equally well established. Examples of this latter use include multipaned windows and doors made up of at least two panes of glass sealed at their peripheral edges to form an insulating chamber therebetween. Such chambers, in this respect, are often made by evacuating the air from the chamber, sealing the glass panes at their edges and filling the chamber formed with a gas other than air, such as argon.
Important to the acceptance of solar management glasses, including IG units, in the marketplace are the following characteristics which relate directly to the sputter-coated layer system employed:
1) the desired amount of visible transmittance coupled with an acceptable level of infrared radiation reflectance; PA1 2) a non-mirror-like appearance (i.e. a low visible "reflectance" as defined below); and PA1 3) a substantially neutral visible reflected color when viewed from the glass side (i.e. a color falling within the range of from colorless to slightly blue). PA1 a) an undercoat layer comprised of metal oxide or nitride having an index of refraction of about 2.35-2.75; PA1 b) a layer comprised of metallic silver, and PA1 c) an overcoat layer comprised of a metal oxide or nitride having an index of refraction of about 1.85-2.25; PA1 T.sub.vis =84%-90% PA1 R.sub.s =4.5 to 6.5 (ohms/sq.) PA1 E.sub.n =0.04-0.07
In addition to these characteristics, the coating system employed must be economical to produce. If it is not, the ultimate product, such as in an IG unit, may become so expensive as to inhibit demand.
It is well-known in the art that these desired characteristics often conflict when attempting to achieve them, and that, therefore, trade-offs often become necessary. For example, simultaneous achievement of very high levels of visible transmittance with an acceptably high level of IR (infrared) reflection may either become impossible to achieve, or only possible with undesirably high levels of reflectance and unacceptable colors. This problem is particularly acute in IG units where, for example, three panes of glass are employed. While three pane IG units allow for increased insulation, and a layer system on two or more internal surfaces, the use of more than one layer has in the past, prior to this invention, resulted in either too low a resultant visible transmittance or, to achieve the necessary level of visible transmittance, a less than optimal IR reflectance.
In other trade-offs, undesirable colors and mirror-like windows (or doors) become unavoidable. In still further trade-offs, cost of production becomes a significant factor.
The above-described problems create a need in the art for a new sputter-coated layer system which can achieve a better balance among these characteristics, and finds particular utility not just on monolithic substrates but in two, three, or more pane IG units.
In recent years the use of Si.sub.3 N.sub.4 in various layer systems has become known and the subject inventors along with others have created various, commercially, acceptable layer systems employing one or more layers of Si.sub.3 N.sub.4, often with silver, to achieve relatively high levels of IR reflectance, such that the resulting glasses are appropriately referred to as "low-E" glasses. Such glasses, however, have not been able to achieve the high levels of visible light transmittance exhibited in this invention. Examples of such glasses are found in U.S. Pat. Nos. 5,376,455; 5,344,718; and 5,377,045 just to name a few. Moreover, combinations of silver, nickel and chromium with TiO.sub.2 have been employed to achieve low-E glasses. See U.S. Pat. No. 5,563,734.
Aesthetically, both mirror-like and purple color qualities may eliminate the marketability of any product exhibiting these characteristics. Loss of visible transmittance while undesirable, does not become truly objectionable until, in a monolithic sheet, it drops below about 70% and in an IG unit it drops below about 63%. However, in certain uses, where low shading coefficients are not required, it is desirable for commercial purposes to have the visible transmission of a monolithic sheet at least about 84% while achieving at the same time a high IR reflectance, as represented by a sheet resistance (R.sub.s) of less than or equal to about 5.5 ohms/sq. and a normal emissivity (E.sub.n) of less than or equal to about 0.065, all while maintaining a substantially neutral color as viewed from both the film side and glass side of the substrate, coupled with a non-mirror-like reflectance characteristic.
In U.S. Pat. No. 5,302,449 there is reported a rather complex layer system as well as its presumed commercial counterpart in IG unit form. Commercially, the system is known as Cardinal 171 sold by Cardinal IG Company. The layer system as taught in this patent varies the thicknesses and types of materials in the layer stack to achieve certain solar management qualities, as well as employing an overcoat of an oxide of zinc, tin, indium, bismuth, or oxides of their alloys including the oxide of zinc stannate, to achieve abrasion resistance. In addition, the system employs one or two layers of gold, copper or silver to achieve its end results. When two layers of silver are used it is said that the first is between 100 .ANG.-150 .ANG. and preferably about 125 .ANG. in thickness while the second, based thereon, is to be between 125 .ANG.-175 .ANG.. When only one silver layer is employed, it is taught that its thickness is to be about 100 .ANG.-175 .ANG., and preferably 140 .ANG..
In actual commercial practice, the aforesaid Cardinal IG units have been found to achieve quite acceptable color characteristics and relatively good non-mirror-like visible reflectance. However, this otherwise quite acceptable IG system has been found to have visible transmittance properties less than 75%.
It has also been known, prior to our invention, to use various combinations of metallic and oxide layers of such elements as tin, zinc, silver, indium, aluminum, titanium, chromium, nickel, magnesium, silicon nitride, and the like, in order to achieve certain desired combinations of solar management properties. For example, in U.S. Pat. No. 4,548,691 a high visible transmittance layer system is disclosed (e.g. T.sub.vis 85-86%). Such a layer system from the glass outwardly generally comprises: SnO.sub.2 or In/Ag/Al, Ti, Zr, Cr or Mg/SnO.sub.2 or In.
The silver layer is said to be on the order of 50-150 .ANG. thick. In practice, such a system, while having high visible light transmittance, unfortunately, exhibits a light purple coloration, and has a rather high sheet resistance of about 6.7-8.2 ohms/square. This, coupled with the thinness of its silver layer, manifests itself in a rather low level of IR reflectance in the ultimate layer system provided.
Another layer coating system known prior to our invention consists from the glass substrate outwardly: SnO.sub.2 /ZnO/Ag/ZnO/SnO.sub.2. In this coating system, ZnO is used to reduce sheet resistance (Re) and emittance (En) by, it is believed, providing a smoother surface on which to deposit the silver (i.e. providing a nucleating layer for the silver apparently superior than the known use of other known nucleating materials). Moreover, the visible light transmittance (T.sub.vis) is an acceptably high 84% (R, is about 5.3 ohms/sq. and E.sub.n is 0.060). Unfortunately, the color manifested by this layer system is an undesirable purple.
In yet another type of layer coating system known prior to our invention, and reported in German published Application DE 19520843A1, a sub-stoichiometric metallic oxide layer (e.g. ZnO.sub.x, ZnTaO.sub.x, TaO.sub.x) is employed as an essential layer beneath the silver layer to increase the conductivity of the Ag layer in a system described generally as, from the substrate outwardly: EQU substrate/oxide/sub-oxide/Ag/blocker/oxide.
A doubling of the system is also contemplated. According to this published disclosure, the oxide layers are transparent anti-reflective layers (e.g. Bi Al-oxide, Sn Mg-oxide, etc.), while the blocker or barrier layer is an adhesion mediating layer of metal or sub-oxide (e.g. oxide or sub-oxide of Ti, Cr, Nb) serving to protect the Ag layer from aggressive environmental atmospheres. It is said that through the use of a special sub-oxide layer (e.g. ZnO.sub.x, TaO.sub.x etc.) beneath the silver layer, the conductivity of the silver is enhanced as much as 30%, leading to the combination of a low-E and high visible transmission layer coating system. As demonstrated in this published disclosure, the necessity of having to form a sub-stoichiometric metallic oxide layer beneath the Ag layer(s), in order to achieve the desired emissivity and visible transmission values over the then known prior art system, adds an undesirable complexity to the manufacturing process.
In still another prior art, commercial product sold by our assignee (and constituting our invention), a first layer of TiO.sub.2 is employed. However, thereafter, the layer system is quite distinct from this invention since this prior art system employs both Si.sub.3 N.sub.4 and nichrome layers to surround the silver. Moreover, while an excellent layer system for many monolithic and IG unit applications, it does not achieve the very high T.sub.vis characteristics of this invention and, thus, is not, for example, useful in all IG applications where three panes are used or high visible light transmittance is required.
In view of the above, it is apparent that there exists a need in the art for a high visible light transmitting, high IR reflecting layer system which also, preferably is durable, substantially neutral in color, does not require the additional step of forming a sub-stoichiometric metallic oxide layer beneath the metallic conductive layer (e.g. beneath a silver layer), does not exhibit a significant purple tint, and does not have a mirror-like appearance. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure: